Amusingly the Expanse was originally designed by Ty Franck as an MMO setting, then when that didn't pan out he used it as a tabletop RPG setting for his home game, (using the d20 Modern ruleset) which then led one of his players, Daniel Abraham, to say "Hey! We should really use this setting to write a book you know". The rest is history.
Of course, there's an official Expanse tabletop RPG now: https://greenroninstore.com/collections/the-expanse-rpg
I knew Franck designed it as an RPG, didn't know that was how Abraham got involved though. Someone has to jump in with a decent PC game at some point surely.